The present invention relates to a liquid crystal display device, and more specifically, to a liquid crystal display device in which three cholesteric liquid crystal display layers of selective reflection type are laminated.
Recently, the technical field of electronic paper that can maintain a display without a power supply and can be electrically rewritten has been rapidly developed. Electronic paper is aimed at realizing extremely low power consumption, capable of memory display even if the power supply is turned off, a reflective display gentle on the eye and which does not tire ones eyes, and a flexible, thin display device like paper. Applications for use in electronic books, electronic newspapers, electronic posters, etc., are being developed. As display systems, an electrophoresis system in which charged particles are moved in air or liquid, a twist ball system in which charged particles classified by two colors are rotated, and a bistable cholesteric liquid crystal system of a selective reflection-type that utilizes the interference reflection of a liquid crystal layer are being developed.
Among these various systems, the cholesteric liquid crystal system is overwhelmingly advantageous in producing a color display. In systems other than the cholesteric liquid crystal system, color filters classified by three colors need to be arranged for each pixel, and therefore, the brightness is ⅓ at the maximum, which corresponds to three divisions, and which are not practical. In the cholesteric liquid crystal system, colors are reflected by the interference of liquid crystals, and therefore, a color display can be produced just by lamination, and there is an advantage in that a brightness of nearly 50% or more can be obtained. A color display device that uses the cholesteric liquid crystal system is described in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-116461.
FIG. 1A and FIG. 1B are diagrams explaining the principle of a display using cholesteric liquid crystals. As shown schematically, the panel has a configuration in which a liquid crystal layer 1 is sandwiched and held between transparent substrates 3 and 4. Substrate 3 is a substrate on the device observation side. On the surface outside substrate 4, a black light absorbing layer 5 is provided. The cholesteric liquid crystal has two stable states: one state in which layers are in parallel to the substrate surface; and another state in which layers are in the vertical direction. The two states can be switched electrically and are characterized by bistability by which the two states can be maintained without the supply of power.
As shown in FIG. 1A, when a high voltage is applied to an electrode (not shown) provided on substrates 3 and 4, the screw axes of liquid crystal molecules 2 linked spirally are oriented in the direction vertical to substrates 3 and 4 and a state is brought about in which layers are parallel to the substrate surface. This state is called a planar state. In the planar state, an interference reflection phenomenon occurs in accordance with the pitch of the layer, exhibiting a specific color and a reflective display is produced. This is a light state (reflective state). As shown in FIG. 1B, when low voltage is applied to the electrode provided on substrates 3 and 4, a state is brought about in which the screw axes of liquid crystal molecules 2 linked spirally are oriented in the direction parallel to substrates 3 and 4. This state is called a focal conic state. In the focal conic state, interference reflection does not occur, and therefore, light incident to the device is transmitted and absorbed by light absorbing layer 5 of substrate 4. This is a dark state (transmitting state). In the reflective state, the light that is not reflected is just transmitted through the liquid crystal layer, and it is therefore, possible to synthesize a reflective color by arranging liquid crystal layers that reflect different colors in the lower layer.
Because of interference reflection, the light reflected in the light state differs depending on the wavelength. Because of this, it is possible to obtain panels of reflected light from which exhibit red (R), green (G), and blue (B) by setting the screw pitch of the liquid crystal.
FIG. 2 is a diagram showing an outline of a color cholesteric liquid crystal display apparatus that has a color enabled display by laminating three panels. As shown schematically, in the order from the observation side, a blue (B) panel 10B, a green (G) panel 10G, and a red (R) panel 10R are laminated and thus a liquid crystal display device 9 is configured. A drive circuit 11 is connected to the electrode of each panel via flexible cables 12B, 12G, 12R. By applying voltage to the electrode of each panel from drive circuit 11, it is possible to bring a cell corresponding to the electrode of each panel into a light state and dark state and thus an image can be displayed. Each panel comprises a matrix electrode and can produce a dot matrix display.
FIG. 3 is a diagram showing a sectional view of liquid crystal display device 9 in FIG. 2. The electrode is not shown schematically. Each of panels 10B, 10G, 10R has a configuration in which each of liquid crystal layers 1B, 1G, 1R is sandwiched and held between transparent substrates 3 and 4 and the liquid crystal layer is sealed by a seal 6. Panels 10B, 10B, 10R are arranged in the order from the device observation side and panels 10B, 10G are bonded by a first adhesive layer 7 and panels 10G, 10R are bonded by a second adhesive layer 8. On the surface outside the substrate on the opposite side of the device observation side of panel 10R, black light absorbing layer 5 is provided. In the following explanation, panel 10B on the device observation side is referred to as a first (blue) panel and its liquid crystal layer 1B as a first (blue) liquid crystal layer, panel 10G next to the first panel is referred to as a second (green) panel and its liquid crystal layer 1G as a second (green) liquid crystal layer, and panel 10R next to the second panel is referred to as a third (red) panel and its liquid crystal layer 1R as a third (red) liquid crystal layer.
If first panel 10B is brought into a light (reflective) state and second and third panels 10G, 10R are brought into a dark (transmitting) state, a blue display is produced. Similarly, if second panel 10G is brought into a light (reflective) state and first and third panels 10B, 10R are brought into a dark (transmitting) state, a green display is produced, and if third panel 10R is brought into a light (reflective) state and first and second panels 10B, 10R are brought into a dark (transmitting) state, a red display is produced. Further, if first and second panels 10B, 10G are brought into the light (reflective) state and third panel 10R is brought into the dark (transmitting) state, a cyan display is produced, if second and third panels 10G, 10R are brought into the light (reflective) state and first panel 10B is brought into the dark (transmitting) state, a yellow display is produced, and if first and third panels 10B, 10R are brought into the light (reflective) state and second panel 10G is brought into the dark (transmitting) state, a magenta display is produced. If all of first to third panels 10B, 10G, 10R are brought into the light (reflective) state, a white display is produced and if all of first to third panels 10B, 10G, 10R are brought into the dark (transmitting) state, a black display is produced.
The selective reflection system of a cholesteric liquid crystal of lamination type has a problem of color purity of a red display. FIG. 4 is a diagram explaining the phenomenon of the drop in color purity due to the shift of the reflection wavelength band. The principle of the selective reflection of a cholesteric liquid crystal of lamination type is the interference reflection of a cholesteric liquid crystal in a multilayer state and the reflection wavelength band shifts depending on the angle of incident light to a liquid crystal display device from outside. With the increasing angle from the vertical incidence, the reflection wavelength band shifts toward the shorter wavelength side. Because the display device of reflection type uses light of the environment illumination, such as an electric lamp on site, light with a variety of angles enters the display device. Because of this, as shown in FIG. 4, in liquid crystal panel 10R set to a red display, the green on the shorter wavelength side is mixed in the display color. If green with high visual sensitivity mixes with red even slightly, the purity of red drops and red with a high purity cannot be displayed. Conventionally, coloring matter that absorbs green is mixed in third liquid crystal 1R of third red panel 10R so that green is not reflected in the panel structure in FIG. 3. Because this coloring matter absorbs and eliminates green which would produce noise, the purity of red is enhanced. However, if coloring matter is mixed in a liquid crystal, there arises a problem, such as deterioration of a liquid crystal, a rise in drive voltage, etc.